Mainframes, super computers, mass storage systems, workstations and very high resolution display subsystems are frequently connected together to facilitate file and print sharing. Common networks and channels used for these types of connections may limit system performance by placing restraints on data flow rates, especially in cases where the data is in a large file format typical of graphically-based applications.
There are two basic types of data communications connections between processors, and between a processor and peripherals. A "channel" provides a direct or switched point-to-point connection between communicating devices. The channel's primary task is merely to transport data at the highest possible data rate with the least amount of delay. Channels typically perform simple error correction in hardware. A "network," by contrast, is an aggregation of distributed nodes (e.g., workstations, mass storage units) with its own protocol that supports interaction among these nodes. Typically, each node contends for the transmission medium, and each node must be capable of recognizing error conditions on the network and must provide the error management required to recover from the error conditions
One type of communications interconnect that has been developed is Fibre Channel. The Fibre channel protocol was developed and adopted as the American National Standard for Information Systems (ANSI). See Fibre Channel Physical and Signaling Interface, Revision 4.2, American National Standard for Information Systems (ANSI) (1993) for a detailed discussion of the fibre channel standard. Briefly, fibre channel is a switched protocol that allows concurrent communication among workstations, super computers and various peripherals. Fibre channel is capable of transmitting frames at rates exceeding 1 gigabit per second in both directions simultaneously. It is also able to transport commands and data according to existing protocols such as Internet protocol (IP), small computer system interface (SCSI), high performance parallel interface (HIPPI) and intelligent peripheral interface (IPI) over both optical fiber and copper cable.
Essentially, the fibre channel is a channel-network hybrid, containing enough network features to provide the needed connectivity, distance and protocol multiplexing, and enough channel features to retain simplicity, repeatable performance and reliable delivery. Fibre channel allows for an active, intelligent interconnection device known as a fibre channel switch to connect devices. The fibre channel switch includes a plurality of fabric-ports (F.sub.-- ports) that provide for interconnection and frame transfer between a plurality of node-ports (N.sub.-- ports) attached to associated devices that may include workstations, super computers and/or peripherals. The fibre channel switch has the capability of routing frames based upon information contained within the frames. The N.sub.-- port manages the simple point-to-point connection between itself and the fabric. The type of N.sub.-- port and associated device dictates the rate that the N.sub.-- port transmits and receives data to and from the fabric. Transmission is isolated from the control protocol so that different topologies (e.g. , point-to-point links, rings, multidrop buses, cross point switches) can be implemented.
The Fibre Channel industry standard also provides for several different types of data transfers. A class 1 transfer requires circuit switching, i.e., a reserved data path through the network switch, and generally involves the transfer of more than one frame, oftentimes numerous frames, between two identified network elements. In contrast, a class 2 transfer requires allocation of a path through the network switch for each transfer of a single frame from one network element to another.
Frame switching for class 2 transfers is more difficult to implement than class 1 circuit switching as frame switching requires a memory mechanism for temporarily storing incoming frames prior to their routing to another port. A memory mechanism typically includes numerous input/output (I/O) connections with associated support circuitry. Additional complexity and hardware is required when channels carrying data at different bit rates are to be interfaced.
The allocation of receive buffers on the F.sub.-- port is typically dictated by the maximum number of frames that the N.sub.-- port can send to the F.sub.-- port without an indication that additional receive buffers are available. Unfortunately, a fibre channel switch may be waiting to forward frames to a destination N.sub.-- port that is currently busy and will not be able to accept additional frames which would be otherwise routable. It would be advantageous if the N.sub.-- port could send frames to the fibre Channel switch even if the maximum number of frames received by the fibre channel switch has reached a defined maximum value.